1. Field of the Invention
The invention relates generally to the field of drilling wellbores through subsurface rock formations. More specifically, the invention relates to techniques for safely drilling wellbores through rock formations using an annular pressure control system with a precise wellbore fluid outlet control.
2. Background Art
A drilling system and methods for control of wellbore annular pressure are described in U.S. Pat. No. 7,395,878 issued to Reitsma et al. and incorporated herein by reference. The system generally includes what is referred to as a “backpressure system” that uses various devices to maintain a selected pressure in the wellbore. Such selected pressure may be at the bottom of the wellbore or any other place along the wellbore.
An important part of the system described in the '878 patent as well as other systems used to maintain wellbore annulus pressure is a controllable flow area “choke” or similar controllable flow restrictor. The controllable flow restrictor may be actuated by devices such as hydraulic cylinders, electric and/or hydraulic motors or any other device used to move the active elements of a controllable flow restrictor.
In the case of hydraulic cylinders used as actuators, for example, one issue that is not effectively addressed is the tradeoff between speed of operation of the actuator, and the accuracy of control. Speed of operation of the actuator may be increased by increasing the control pressure or by increasing the actuator piston surface area. With such increase in operating speed, it becomes increasingly difficult to precisely control the position of the actuator in response to pressure variations in the wellbore. “Overshoot” and “undershoot” of the actuator from the instantaneously correct position is common. Conversely, if the actuator operating speed is reduced by reducing the operating pressure or decreasing the piston surface area, it is possible to make the actuator operate too slowly to response to rapid wellbore pressure variations.
Accordingly, there is a need for a more effective actuator for controllable flow restrictors that does not require a tradeoff between speed of operation and accuracy of position control.